A known magnetic sensor device includes a magnetoresistance effect element and a signal amplification IC that are enclosed in a casing and mounted on a conveyance-path-side surface of a magnet. A cover for covering the magnetoresistance effect element and the signal amplification IC is provided on a conveyance path side of the casing to protect the magnetoresistance effect element and the signal amplification IC. That is, the magnetoresistance effect element and the signal amplification IC are disposed between the cover and the magnet, for example as in Patent Literature 1.